Method for preparing an aqueous hydrogen peroxide solution directly from hydrogen and oxygen

ABSTRACT

A catalytic process and a device for preparing, in absolute safely, aqueous hydrogen peroxide solutions at high concentration levels directly from hydrogen and oxygen; more particularly, a method whereby hydrogen and oxygen are injected, into the aqueous medium in proportions corresponding to the flammability range of the hydrogen-oxygen mixture, and are present in proportions outside the flammability range in the continuous gas phase. The invention also concerns a device for implementing the method.

The present invention relates to a catalytic process and to a device forpreparing, in perfect safety, aqueous hydrogen peroxide solutions athigh concentrations directly from hydrogen and oxygen. Moreparticularly, a subject-matter of the invention is a process in whichhydrogen and oxygen are injected into the aqueous medium in proportionscorresponding to the flammability range of the hydrogen-oxygen mixtureand are present in proportions outside the flammability range in thecontinuous gas phase. Another subject-matter of the invention is adevice for the implementation of the process.

The hydrogen and oxygen gas mixture is known to be flammable, evenexplosive, when hydrogen is present at molar concentrations of between 4and 94% under standard temperature and pressure conditions, that is tosay when the ratio of the hydrogen molar concentration to the oxygenmolar concentration is greater than 0.0416 (Encyclopédie des Gaz[Encyclopedia of Gases], Air Liquide, page 909).

To avoid any risk of explosion or fire, it is recommended either tooperate with a hydrogen/oxygen ratio below the lower flammability limitor to use an inert gas, such as nitrogen, argon, helium or neon (U.S.Pat. Nos. 4,681,751, 4,009,252, EP 0,787,681).

In point of fact, to obtain satisfactory results, it is necessary towork with a hydrogen/oxygen ratio situated in the flammability range.Thus, the document U.S. Pat. No. 4,009,252 discloses a hydrogen/oxygenmolar ratio of between 1/20 and 1/1.5 and preferably of between 1/10 and1/2. Likewise, the document U.S. Pat. No. 4,336,239 teaches the readerto operate in the presence of a hydrogen/oxygen molar ratio of less than0.2 and preferably of between 1/15 and 1/12.

The term “direct synthesis of an aqueous hydrogen peroxide solution” isunderstood to denote the synthesis of hydrogen peroxide from hydrogenand oxygen in an aqueous medium comprising a catalyst.

The direct synthesis of an aqueous hydrogen peroxide solution,continuously or batchwise, in a stirred reactor has formed the subjectof many studies. The reactor generally comprises an aqueous region,occupied by the working solution and the catalyst, and a region,occupied by the gases, situated above the aqueous region. It is equippedwith a stirring system which makes it possible both to stir the aqueousregion and to disperse the gases in the aqueous phase. The reactants,namely the hydrogen and the oxygen, and the inert gases are injectedinto the region of the gases.

The term “working solution” is understood to denote the aqueous medium,comprising water, acids and optionally decomposition inhibitors orstabilizers for hydrogen peroxide, in which the hydrogen peroxide isformed.

It has been observed that when the direct synthesis of an aqueoushydrogen peroxide solution is carried out in a stirred reactor asdescribed above, the catalyst, thrown under the effect of the stirringon to the walls of the reactor and the shaft of the stirrer which aresituated in the region of the gases, would be in direct contact with thereactants. During the synthesis, the catalyst particles in the region ofthe gases will dry out and will spontaneously bring about the ignitionof the hydrogen-oxygen gas mixture if the molar concentration of thehydrogen is greater than 0.04.

This is why, in Example 1 of document U.S. Pat. No. 4,279,883, whichillustrates the direct continuous synthesis of an aqueous hydrogenperoxide solution in a stirred reactor, a hydrogen, oxygen and nitrogengas mixture are introduced continuously into the gaseous region of thereactor, so that the hydrogen, oxygen and nitrogen partial pressures inthe gases collected at the outlet are maintained respectively at 5, 49and 113 atmospheres, that is to say a hydrogen molar concentration of3%. The industrial manufacture of an aqueous hydrogen peroxide solutionunder the safety conditions according to the document U.S. Pat. No.4,279,883 is economically out of the question, however, given the lowconcentration of the aqueous hydrogen peroxide solution obtained.

In order to be usable, this aqueous solution requires an additionalconcentration stage.

The direct synthesis of an aqueous hydrogen peroxide solution can alsobe carried out in a tubular reactor composed of a long pipe (pipeline)filled with working solution in which the catalyst is suspended and intowhich gaseous oxygen and hydrogen are injected in the form of smallbubbles in proportions above the lower flammability limit of thehydrogen-oxygen mixture (U.S. Pat. No. 5,194,242). The safety of such aprocess is only assured provided that the gaseous reactants aremaintained in the reactor in the form of small bubbles. According to thedocument U.S. Pat. No. 5,641,467, the latter can only be obtained with ahigh rate of circulation of the working solution.

A catalytic process and a device have now been found which make itpossible to prepare aqueous hydrogen peroxide solutions at highconcentrations directly from hydrogen and oxygen in a stirred reactor inperfect safety and economically.

This process is characterized in that hydrogen and oxygen are injected,in the form of small bubbles, into the lower part of the aqueousreaction medium, which has been rendered acidic by the addition of amineral acid and which comprises a catalyst in the dispersed state, withmolar flow rates such that the ratio of the hydrogen molar flow rate tothe oxygen molar flow rate is greater than 0.0416 and in that oxygen isintroduced into the continuous gas phase and/or into the upper part ofthe aqueous reaction medium in an amount such that the molar ratio ofhydrogen to oxygen in the continuous gas phase is less than 0.0416.

The term “small bubbles” is understood to denote bubbles with a meandiameter of less than 3 mm.

The injections of hydrogen and of oxygen in the form of small bubblesinto the lower part of the aqueous reaction medium are preferablysituated at the bottom of the stirred reactor and are preferablycontagious, in order for the H₂ and O₂ bubbles to mix together asquickly as possible.

Mention may be made, as mineral acid, of, for example, sulphuric acidand orthophosphoric acid.

The aqueous reaction medium can additionally comprise stabilizers forhydrogen peroxide, such as, for example, phosphonates or tin, anddecomposition inhibitors, such as, for example, halides. The bromide isthe particularly preferred inhibitor and it is advantageously used incombination with bromine in the free state (Br₂).

According to the invention, the oxygen injected, in the form of smallbubbles, into the lower part of the aqueous reaction medium and theoxygen introduced into the continuous gas phase and/or into the upperpart of the aqueous reaction medium can additionally comprise hydrogenin an amount such that the ratio of molar concentration of hydrogen tomolar concentration of oxygen is less than 0.0416.

According to the present invention, the operation can be carried outjust as easily continuously as semi-continuously.

The oxygen feed in the form of small aqueous into the lower part of theaqueous reaction medium can be provided in all or in part by the gaseouseffluent at the outlet of the reactor.

It is also possible to use the gaseous effluent at the outlet of thereactor to feed the continuous gas phase and/or the upper part of theaqueous reaction medium. In this case, the composition of the gaseouseffluent can be adjusted by addition of oxygen and optionally by removalof hydrogen, so that the ratio of the molar concentration of hydrogen tothe molar concentration of oxygen in the continuous gas phase is lessthan 0.0416.

The catalyst generally used comprises at least one element chosen frommetals from Group IB and VIII of the Periodic Table. Advantageously,gold, platinum, palladium and ruthenium are chosen. Use is preferablymade of palladium, platinum or the palladium-platinum combination orbetter still palladium or the palladium-platinum combination.

In the case of a palladium-platinum composite catalyst, the platinumpreferably represents between 1 and 50% of the total weight of themetals and better still approximately 2%.

According to the invention, the catalyst can also be supported. Thesupports generally used are, for example, silica, alumina,silica-aluminas and titanium dioxide.

The supported or non-supported catalyst is generally suspended in theaqueous reaction medium. Use is preferably made of a supported catalystand better still of a supported catalyst comprising between 0.2 and 2%by weight of metal or metals with respect to the support.

The temperature and the pressure prevailing inside the reactor areadjusted in order to optimize the selectivity of the reaction withrespect to hydrogen and the productivity with regard to hydrogenperoxide.

The temperature is generally between 0 and 60° C. and preferably between5 and 30° C.

The pressure prevailing inside the reactor is generally aboveatmospheric pressure and preferably between 30 and 100 bar andadvantageously between 40 and 60 bar.

The ratio of the molar flow rate of hydrogen to the molar flow rate ofoxygen which are injected into the lower part of the aqueous reactionmedium can vary within wide limits. It is preferably between 0.05 and 5and more particularly between 0.2 and 1. A molar ratio in the region of0.3 is advantageously used.

When the operation is carried out semi-continuously, all of the workingsolution and all the catalyst are introduced into the reactor before thebeginning of the direct synthesis and the hydrogen and the oxygen areintroduced continuously.

It is also possible to feed the reactor continuously with the workingsolution, to which the catalyst has been added, and to introduce thehydrogen and the oxygen continuously. In this case, the solutioncomprising the hydrogen peroxide formed is extracted continuously fromthe reactor.

The catalyst is subsequently separated, by filtration of the finalsolution comprising the hydrogen peroxide formed under semi-continuousconditions or of the hydrogen peroxide solution extracted continuouslyfrom the reactor, and then optionally reintroduced into the reactor.

When the reactor is equipped with a filter, the catalyst is heldpermanently in the reactor and the hydrogen peroxide solution isextracted and filtered simultaneously.

Another subject-matter of the invention is a device which makes itpossible to manufacture, in perfect safety and economically,concentrated aqueous hydrogen peroxide solutions directly from hydrogenand oxygen. This device, comprising a stirred reactor fed continuouslyor non-continuously with working solution, is characterized in that thereactor is equipped (i) with one or more inlet(s) for gaseous hydrogen,in the form of small bubbles, into the lower part of the aqueousreaction medium; (ii) with one or more inlet(s) for gaseous oxygenoptionally comprising hydrogen, in the form of small bubbles, into thelower part of the aqueous reaction medium, the inlet(s) for oxygen intothe liquid phase are preferably contiguous with that (those) forhydrogen, in order for the H₂ and O₂ bubbles to mix together quickly,(iii) with a pressure regulator which makes it possible to keep thepressure prevailing inside the reactor constant by discharging excessunconsumed gaseous reactants; and (iv) with one or more inlet(s) forgaseous oxygen optionally comprising hydrogen into the continuous gasphase and/or into the upper part of the aqueous reaction medium, whichis/are controlled by an analyser of the gas flow exiting from thereactor, so that the molar ratio of hydrogen to oxygen in the continuousgas phase is less than 0.0416.

The reactor is equipped with an outlet which makes possible thecontinuous or semi-continuous extraction of the aqueous hydrogenperoxide solution. This outlet is optionally equipped with a filterwhich makes it possible to separate the catalyst from the aqueoushydrogen peroxide solution.

According to the invention, the gas flow exiting from the reactor can bereinjected into the circuit feeding the lower part of the aqueousreaction medium with oxygen. This gas flow, after optional adjustment ofthe hydrogen content by addition of oxygen and optionally by removal ofhydrogen, for example by using a membrane, can also be reinjected intothe circuit feeding the continuous gas phase with oxygen and/or theupper part of the aqueous reaction medium. The hydrogen thus separatedcan be reinjected into the lower part of the aqueous reaction medium.

Preferably, at least one inlet for hydrogen and at least one inlet foroxygen, in the form of small bubbles, into the lower part of the aqueousreaction medium are situated at the bottom of the stirred reactor.

The reactor can be an autoclave of cylindrical, cylindroconical orspherical shape stirred by means of a vertical shaft equipped with oneor more impellers or one or more turbines.

Generally, any reactor commonly used when a suspended catalyst isinvolved and which is capable of providing good heat exchange and ofmaintaining the gaseous reactants of the reaction in the form of a cloudof larger possible number of small bubbles may be suitable.

The stirring can also be provided by several independent impellers orturbines each driven by a stirrer shaft which is attached to the bottomor to the lid or to the sides of the reactor. The turbine situated inthe upper part of the aqueous reaction medium can be of the“self-suction” type, that is to say that it sucks the continuous gasphase of the reactor from the stirrer shaft, which is hollow, and thendiffuses this gas phase into the aqueous reaction medium, or of“flanged” type.

The stirring can be supplemented by devices commonly used to render thestirring highly efficient, such as, for example, one or more bafflespositioned vertically or radially.

Use is generally made of heat exchangers, such as tubular coils, bundlesof vertical pipes or else set of radial vertical plates or else woundspirals, in order to provide for the regulation of the temperature ofthe reaction medium. These exchangers are preferably situated inside thereactor. Use is advantageously made of a vertical tubular bundle orwound spirals or a bundle of vertical plates positioned radially.

The temperature of the mixture can also be regulated by using a jacketedreactor with circulation of water.

The reactor according to the invention is designed so that, if thestirring is accidentally halted, all the gas bubbles can rise and reachdirectly the continuous gas phase solely under the action ofgravitational forces. The various devices installed inside the reactorin order to provide for the heat exchanges and/or the stirring must notform an obstacle to the rise of bubbles and must not result in theformation of pockets of gas inside the aqueous reaction medium.

The reactor can be composed of any material compatible with thereactants used. Use may be made, for example, of metals, such asstainless steels of 304L or 316L type of Hastelloy alloys, or elsemetals coated with chemically resistant polymers, such as PVDF(polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA(copolymer of C₂F₄ and of perfluorinated vinyl other) or FEP (copolymerof C₂F₄ and of C₃F₆.

The feeds of oxygen or of hydrogen, in the form of small bubbles, intothe lower part of the aqueous reaction medium can be provided by pipesor plates made of sintered metal or else various types of nozzle whichmake it possible to eject gas at a high rate and thus bring about theformation of many small bubbles.

A device and a block diagram illustrating an embodiment of the processof the present invention, which are represented in the single Figure,are described hereinbelow.

The device comprises a reactor stirred by means of a vertical shaftequipped with a self-suction turbine a and with a turbomixer b. Onstart-up: the reactor comprises the catalyst suspended in the workingsolution, the combined mixture being brought to the reactiontemperature.

the oxygen introduced into the continuous gas phase at 3 originates fromthe flow 8, that is to say from non-recycled oxygen, and,

the hydrogen injected at the bottom of the reactor is fed via 2.

A pressure regulator i makes it possible to keep the pressure prevailinginside the reactor constant by discharging excess unconsumed gaseousreactants 9. Likewise, the temperature of the reaction medium is keptconstant by virtue of the exchanger e.

The following are continuously introduced into the reactor: at 6, theworking solution; at 2 and 4, hydrogen in the form of small bubbles; at1, oxygen in the form of small bubbles; at 3, oxygen in an amount suchthat the molar concentration of hydrogen in the continuous gas phase isalways less than 4%.

The feed system at 3 is controlled by the in-line analyser g of the gasflow 5 exiting from the reactor. The oxygen feed at 3 is provided by theflow 8 and by the flow 10; the latter originates from the gaseouseffluent at the outlet of the reactor after removal of the hydrogenthrough a membrane s. The hydrogen thus removed provides for the partialhydrogen feed 4 into the lower part of the reaction medium.

The oxygen 1 injected into the lower part of the reaction mediumoriginates entirely from the gaseous effluent at the outlet of thereactor and comprises hydrogen.

All the gas flow rates are regulated using mass flowmeters f. The flowrates for oxygen and for hydrogen injected into the lower part of thereaction medium are such that the ratio of the hydrogen molar flow rateto the oxygen molar flow rate is always greater than 0.0416.

The injection nozzles d make it possible to inject the reactants in theform of small bubbles.

The pump h provides for the recycling of unconsumed hydrogen andunconsumed oxygen.

The aqueous solution comprising the hydrogen peroxide formed issimultaneously separated from the catalyst using the filter c andextracted continuously, 7.

Other specific embodiments are given in the following examples.

EXPERIMENTAL PART Preparation of the Catalyst

The catalyst used comprises 0.8% by weight of metallic palladium and0.04% by weight of platinum, these being supported on a microporoussilica. It is prepared by impregnating silica from the Company Aldrich(ref. 28,851-9), having the following characteristics:

Mean particle size=5 to 15 μm

BET surface=500 m²/g

Pore volume: 0.75 cm³/g

Mean pore diameter: 60 Å

with an aqueous solution comprising PdCl₂ and H₂PtCl₆, followed bydrying and finally by heat treatment while flushing with hydrogen at300° C. for 3 hours.

The Reactor

The reactor is a jacketed stainless steel autoclave with watercirculation and a capacity of 100 cm³, the internal walls of which arecoated with PTFE. It is equipped with a stirrer comprising a verticalshaft with a turbine comprising six radial blades. The reactor is alsoequipped with two inlets made of PTFE capillary tube situated at thebottom of the reactor which make it possible to inject hydrogen andoxygen, in the form of small bubbles, into the lower part of the aqueousreaction medium. It is furthermore equipped with an inlet situated inthe lid of the autoclave which makes it possible to introduce oxygen sothat the molar ratio of hydrogen to oxygen in the continuous gas phaseis always less than 0.0416, that is to say outside the flammabilityrange of the peroxide-oxygen mixture.

The injection of the reactants into the aqueous medium and the injectionof the oxygen into the continuous gas phase are regulated using massflowmeters.

The pressure prevailing inside the reactor is kept constant by virtue ofa discharging device. The hydrogen and the oxygen constituting the gasflow exiting from the reactor are quantitatively determined in line bygas chromatography.

Preparation of an Aqueous Solution (I)

An aqueous solution is prepared by addition of 0.5 g [lacuna] H₃PO₄, 2.5g [lacuna] H₂SO₄ and 50 mg of sodium bromide to 1000 cm³ of distilledwater and 5 mg of Br₂ in the form of 1% bromine water.

General Procedure

50 g of the aqueous solution (I) and 0.3 g of catalyst are introducedinto the autoclave and then the aqueous reaction medium is brought toand maintained at the desired temperature. The inlet for oxygen into thecontinuous gas phase is subsequently opened. The pressure in theautoclave increases to the chosen value and it is then kept constant byvirtue of the pressure regulator.

The hydrogen and the oxygen are subsequently injected into the aqueousreaction medium in the chosen proportions and then the hydrogen in thegas flow exiting from the pressure regulator is quantitativelydetermined every 10 minutes.

After the desired reaction duration, the inlet for hydrogen and oxygeninto the aqueous reaction medium is turned off and the injection ofoxygen into the continuous gas phase is maintained until hydrogen hascompletely disappeared from the latter. The inlet for oxygen is thenturned off, the reactor is then decompressed and, finally, the aqueoushydrogen peroxide solution is recovered.

The aqueous hydrogen peroxide solution recovered is subsequently weighedand then separated from the catalyst by filtration through a filter. Theresulting solution is then quantitatively determined by iodometry, thusmaking it possible to determine the hydrogen peroxide concentration.

The consumption of hydrogen is measured by [lacuna] difference betweenthe amount injected and the amount which has exited from the reactor.

The selectivity of the direct synthesis of hydrogen peroxide withrespect to hydrogen is defined as being the percentage of the number ofmoles of hydrogen peroxide which is formed to the number of moles ofhydrogen which is consumed.

The operating conditions and the results obtained during the varioustests (Examples 1 to 10) are combined in Table I.

TABLE I FLOW RATE FOR H₂ FLOW RATE FOR O₂ RATIO OF INJECTED INTO THEINJECTED INTO THE H₂/O₂ RATES, TEMPERATURE DORATION AQUEOUS MEDIUMAQUEOUS MEDIUM RATES, EXAMPLE (° C.) (HOUR) S1/b S1/b AQUEOUS MEDIUM 120 3 4 0.1 40 2 20 3 4 1 4 3 20 3 4 2 2 4 20 3 4 3 1.3 5 20 3 4 4 1 6 203 4 6 0.67 7 20 3 4 8 0.5 8 12 5 2 0.01 200 9 12 5 2 9 0.2 10  12 5 2 21 FLOW RATE FOR O₂ INJECTED INTO H₂O₂ THE CONTINUOUS RATIO OFCONCENTRATION SELECTIVITY GAS PHASE H₂/O₂ FLOW RATES, OF THE WITHRESPECT EXAMPLE S1/h INJECTED TOTAL SOLUTION % TO H₂ % 1 95 0.042 15 602 95 0.041 17 69 3 94 0.041 18 72 4 93 0.041 17 75 5 92 0.041 17 77 6 900.041 15 81 7 88 0.041 13 82 8 48 0.041 15 41 9 39 0.041 7 88 10  460.041 13 88

EXAMPLES 11 TO 13

Use is made of a cylindrical reactor made of type 316 L stainless steelwith an internal diameter of 98 mm, a height of 200 mm and a totalcapacity of 1500 cm³. The internal walls of the reactor are coated witha PTFE layer with a thickness of 1 millimeter.

Stirring is provided by a vertical shaft equipped with a flangedturbine, the suction of which is directed downwards. The flanged turbinewith a diameter of 45 mm, situated in the middle of the reactor, isequipped with eight blades.

An axial propeller with a diameter of 30 mm, equipped with six inclinedblades, is attached to the end of the vertical shaft close to the bottomof the reactor.

The reactor is also equipped with four vertical baffles and with a heatexchanger with a bundle of 8 vertical tubes in which water circulates at17° C.

The hydrogen and the oxygen are injected into the liquid phase using twostainless steel tubes, the inlets of which are contiguous and aresituated close to the axial propeller.

The procedure of the preceding examples is used, except that 700 g ofaqueous solution (I) and 6 g of catalyst are used.

The operating conditions and the results obtained during the tests(Examples 11 to 13) are combined in Table II.

TABLE II FLOW RATE FOR H₂ FLOW RATE FOR O₂ RATIO OF INJECTED INTO THEINJECTED INTO THE H₂/O₂ RATES, TEMPERATURE DORATION AQUEOUS MEDIUMAQUEOUS MEDIUM RATES, EXAMPLE (° C.) (HOUR) S1/b S1/b AQUEOUS MEDIUM 1121 3 120 300 0.4 12 21 3 80 160 0.5 13 20 3 80 188 0.42 FLOW RATE FOR O₂INJECTED INTO H₂O₂ THE CONTINUOUS RATIO OF CONCENTRATION SELECTIVITY GASPHASE H₂/O₂ FLOW RATES, OF THE WITH RESPECT EXAMPLE S1/h INJECTED TOTALSOLUTION % TO H₂ % 11 2850 0.04 20.1 83 12 1760 0.04 15.3 84 13 17600.039 16.2 85

What is claimed is:
 1. Process for the preparation of an aqueoushydrogen peroxide solution directly from hydrogen and oxygen, whichcomprises: providing a stirred reactor containing an aqueous reactionmedium, wherein the aqueous reaction medium comprises a catalyst in adispersed state and has a lower part and an upper part; adding a mineralacid to the aqueous medium so that the aqueous medium becomes acidic;injecting hydrogen and oxygen, in the form of small bubbles, into thelower part of the aqueous reaction medium a hydrogen molar flow rate andan oxygen molar flow rate, wherein the ratio of the hydrogen molar flowrate to the oxygen molar flow rate is greater than 0.0416; andintroducing oxygen into a continuous gas phase in the stirred reactorand/or into the upper part of the aqueous reaction medium in an amountsuch that the molar ratio of hydrogen to oxygen in the continuous gasphase is less than 0.0416.
 2. Process according to claim 1, wherein theinjections of hydrogen and of oxygen in the form of small bubbles intothe lower part of the aqueous reaction medium are situated at the bottomof the stirred reactor.
 3. Process according to claim 1, wherein theinjections of hydrogen and oxygen into the lower part of the aqueousreaction medium are contiguous.
 4. Process according to any one ofclaims 1 to 3, wherein the reaction medium comprises stabilizers forhydrogen peroxide.
 5. Process according to claim 1, wherein the reactionmedium comprises halides.
 6. Process according to claim 5, wherein thehalide is the bromide.
 7. Process according to claim 6, wherein thebromide is used in combination with bromine in the free state. 8.Process according to claim 1, wherein the catalyst comprises palladium.9. Process according to claim 1 wherein the catalyst comprises platinum.10. Process according to claim 1 wherein the catalyst is supported. 11.Process according to claim 10, wherein the support is chosen fromsilica, alumina and silica-aluminas.
 12. Process according to claim 1,wherein the oxygen, which is introduced into the continuous gas phaseand/or into the upper part of the aqueous reaction medium, compriseshydrogen.
 13. Process according to claim 1, wherein the oxygen, which isinjected into the lower part of the aqueous reaction medium, compriseshydrogen.